The present invention relates to a control system for a hydrostatic transmission.
Infinitely variable transmissions (IVT) include a hydro module having an engine driven variable displacement pump, or variable unit, and a fixed displacement motor, or fixed unit. The variable unit includes a swashplate or yoke which is pivotal about a neutral position. The fixed displacement motor or fixed unit may be connected to vehicle drive wheels through a gear unit and a planetary module. In order to accurately control such an IVT, the position of the swashplate must be accurately controlled with respect to a speed command signal so that the IVT output speed closely matches the commanded speed. It is also desirable to have an IVT control system which operates so that the IVT output speed is equally accurate at slower speeds and at higher speeds, in both forward and reverse operation.
Accordingly, an object of this invention is to provide a control system for an IVT which accurately controls the position of the swashplate with respect to a speed command signal.
A further object of the invention is to provide such an IVT control system which operates so that the IVT output speed closely matches the commanded speed.
A further object of the invention is to provide such an IVT control system which operates so that the IVT output speed is equally accurate at slower speeds and at higher speeds, in both forward and reverse operation.
These and other objects are achieved by the present invention, wherein the hydrostatic module is controlled by a control system which includes a closed loop control algorithm. The control system controls a hydrostatic transmission having an engine driven variable displacement pump coupled to a fixed displacement motor. The pump has a swashplate which is controlled by a control signal generated by the control system. The control system includes a speed command signal generator, a motor speed sensor for generating a sensed motor speed signal, an engine speed sensor for generating an engine speed signal, and a control unit which generates the control signal as a function thereof. The control unit generates a desired motor speed signal as a function of the speed command signal and the engine speed signal, generates a feed-forward command signal as a function of the desired motor speed signal, generates a difference signal as a difference between the desired motor speed signal and the sensed motor speed signal, generates an integral signal by applying an integral function to the difference signal, generating a sum signal as a summation of the feed-forward command signal and the integral signal, and generating the control signal as a function of the sum signal.